This invention relates in general to prnting. More specifically, it relates to web finishing, and in particular to off-line web finishing of pre-printed and rewound webs.
In the manufacture of magazines, mailing inserts, envelopes, brochures and many other printed products, the product is printed on a web of paper, traveling through a printing press at high speed, up to 2,000 feet per minute. In most printing applications, and certainly- those where there is color printing or where the web is run through the press more than once, it is essential to maintain a very precise registration between the web and the printing cylinders acting on the web. This is difficult since paper is elastic and in most modern printing presses such as commercial web offset presses the paper is moistened by ink and water and then heated in dryer. This wetting and drying causes unpredictable variations in the properties of the paper, including its length, which creates a problem in maintaining registration between the web and the equipment acting on it.
In printing presses, the standard approach to maintaining registration has been to stretch the web until it is back in registration, or to hold it in registration against a shrinkage associated with drying. The former technique is the most common approach. For exampIe, in the printing of newspapers with color. The color is first printed on the web, but printed "short", that is, the length of the impression or pattern printed on the web by one revolution of a print cylinder is slightly less than the desired final length. In a second pass, when black ink only is printed on the web, the web is stretched between a pair of draw rolls to the desired full impression length. The web has registration marks printed on it at regular intervals. Optical scanners detect the marks, compare the sensed impression length with the desired value, and produce an electrical control signal. The value and sign of the signal is used to increase or decrease the speed of the downstream roll, and thereby adjust the length of the web. This mode of adjustment, which is perhaps the most widely used, requires a slip.page between the draw roll, e.g. a chill roll following the dryer, and the web, but there can be.no slippage between the print cylinders and the web. In other systems the adjustment is made by changing the path length of the web between sets of draw rolls, as with a dancer roll that moves under control of the registration correction signal.
In U.S. Pat. No. 4,096,801 to Martin the web in a printing press is secured against slippage with respect to all of the rolls. The dryer in the press is assumed to produce a shrinkage of the web. By drawing the web at a uniform speed throughout the press, the web is automatically stretched back to its initial length. In other words, Martin "locks" the printing and draw roll cylinders onto the web and thereby secures the web in a known relationship (registration) with respect to the cylinders operating on it.
Registration is also a very significant problem in web finishing, as opposed to web printing. Web finishing is the processing of a printed web to a finished product such as a multi page "signature" which forms a magazine, or a part of a magazine. The processing often includes folding, perforating, spot application of glue, die cutting and rotary cutting. These functions are usually performed by a series of machines arranged in a line. These operations can be performed "in line", that is, receiving a freshly printed web directly from a printing press, or "off-line", that is, receiving the web from a rewound, pre-printed roll. In recent years finishing has been principally in-line. A principal reason for this is that if the printed web is wound and stored, because the paper is elastic, responsive to environmental conditions such as humidity and temperature, and has been strained by processing, its properties change over time. For finishing, a crucial problem is that once stored the dimensions of the paper change unpredictably and non-uniformly, which of course changes the repeat length of the pattern along the web. The pattern may shrink, expand, or do both within the same rewound web. In-line finishing avoids the problems by not allowing time for the web to change.
In-line finishing has also found favor because prior off-line finishing set preconditions on how the web is printed in order to allow finishing of a rewound roll. A typical precondition is requiring that the web be printed "short" so that it can be stretched back into registration in the finishing line. Ideally, the printing process should be completely independent of the finishing process; any roll from any printing press should be able to be finished along with other rolls from other presses of the same repeat length. This objective is not attainable with current off line systems.
In-line web finishing, however,.has several significant disadvantages. First, it is too slow to be operationally linked to modern printing presses without significant costs. A typical operational speed of a press is up to 2,000 feet per minute, whereas an in line finishing system typically operates at up to 1,000 feet per minute. The in-line web finishing therefore cuts the productivity of the entire printing press about in half. Second, an in-line finishing system has a significant make-ready time, typically 8 to 48 hours, as a series of pieces of equipment a re adjusted to very tight tolerances. While the finishing equipment is made ready, the printing press, which is a substantial capital investment, is idle. This further reduces the productivity of the entire printing operation. In the known newspaper printing system where black ink is applied in a second pass there is only one operation, the printing of black ink; a finishing line will normaly perform 20 to 30 operations on the web in one pass.
Several other design problems have plagued automated finishing operations. One is that the tension used to stretch the web to maintain registration can be sufficient to weaken or even break the web, particularly lightweight webs such as thcse used to form airmail envelopes. Web breaks are costly since some printed material is wasted and because the line is down while the web is refed through the line and registration adjusted. Another problem is maintaining registration despite 1) rapid, often locaI, changes in the repeat length--which requires a fast dynamic response--and 2) accumulating registration errors of the same type (long or short repeat lengths) that cannot be accommodated by registration adjustment mechanisms in the system.
As noted above, in general the prior art solution to the registration problem has been to stretch the web, and therefore increase the tension in the web, until it is in registration. The most widely used arrangement is to have a variable speed draw roll operating under the control of an optical scanner that looks at the registration marks. This system works, but it does not work for light weight paper, it does not have a fast dynamic response time and while it may be acceptable for simple printing and finishing operations, e.g. where the only operation is to print black ink, it is not well suited for use in a high speed finishing line which performs, on average 20 to 30 operations.
With regard to the response time, conventional scanning equipment monitors the web once during the passage of multiple impressions, usually in the range of 10 to 100 depending on factors such as the press or line speed, the size of the impressions, and the capabilities of the monitoring equipment, and the susceptibility of the registration control system to "hunting". In web finishing, there can be significant variations in the registration between these monitorings and there can be cumulative errors which can accumulate to a significant registration error before the situation is monitored, let alone corrected. Moreover, even if one monitors more often, not all control system and adjustment equipment can respond to the rapid variations quickly enough The result can be that the adjustment system hunts but cannot keep up with the corrections required. Also, where the errors are cumulative, the system may not be able to keep up with the ever growing misregistration. With respect to the number of operations performed in a finishing line, the problem is that if the tension in the web is adjusted at one station to produce a correct registration, this change in tension will fight against the registration of the web at other stations where other operations are performed. In short, tension adjustments at one location fight adjustments at another location leading to increased difficulties in maintaining registration throughout the finishing line, and to an increased likelihood. that the tension will reach a level sufficient to break the web.
As noted above, in some systems registration is maintained by adjusting the paper path length as it traverses the printing press or finishing line. A common technique is to pass the web over a movable, pre-loaded idler or "dancer" roll so that changes in registration can be affected by changes in the speed at which the paper is moving with respect to the equipment at different points, which results in changes in the total length of the paper in the press or line. Path length adjustments work for certain applications, but they cannot deal with the accumulating adjustments required for off-line web finishing. For example, if a web should have a repeat (impression) length of 630.0 mm, but is consistently printed long at 630.25 mm, during the passage of 100 impressions, in a few seconds, there is a cumulative misregistration of 25 mm, about one inch. While a path length change can in theory compensate for this cumulative error, it cannot do so indefinitely. In the case of the dancer roll, its travel will eventually reach an extreme limit position and it will be unable to make further compensating movements.
U.S. Pat. Nos. 4,078,490 and 4,085,674 to Biggar compensate for misregistration by changing the phase angle between an output gear (acting through a worm gear) and a line shaft. Registration units operate at each station. In the '674 patent, for example, a registration unit for a die cutting station has a motor that rotates a sleeve relative to a shaft of a first cylinder. This rotation shifts the phase of a drive gear and a die cylinder relative to the first cylinder. There is no apparent control of web tension to hold it at a constant value. There is likewise no way to deal with cumulative errors other than through constant adjustment of the phase angle. While this is theoretically solution in practice known systems cannot keep up with the accumuIation errors that may be encountered in processing rewound webs.
U.S. Pat. No. 4,452,140 to Isherwood et al. describes another system, one using a dancer roll to adjust paper path length, s discussed above. In FIG. 2 Isherwood et al. show a further registration adjustment at a downstream processing station. This further registration can be accomplished by a differential gear assembly to introduce phase angle adjustments. The web is monitored by a single detector. There is no teaching to maintain the tension in the web constant.
U.S. Pat. No. 3,841,216 to Huffmann discloses a system for registration control on a second pass of a printed web, with registration marks, through a printing press or "processing device". Huffmann adjusts first by metering the web at the infeed rolls. Other variations, termed by Huffmann as a "stretch factor", are compensated by a proportional registration shaft Z driven by a differential 106 responsive to sensed registration errors. The signals control signals reflect inputs from an electric eye and an encoder. Rotation of the shaft Z alters the web path length (FIG. 4) and the phase reIation of the blanket cylinders f printing stations in the press. The Hoffmann system also adjusts the feed rate of the web to control registration. These adjustments change in tension in the web. Huffmann provides a hybrid system which controls registration using both adjustments in web tension and in paper path length. However, it is limited in its ability to compensate for cumulative errors to the same extent as the Isherwood path length adjustment system. Also, it is in essence a more sophisticated variation on the standard "stretch into register" approach. The web is pulled to achieve registration.
None of these known systems, whether those described above generally or the specific arrangements disclosed in the patents identified above, have resulted in commercially acceptable off-line web finishing systems. No known system, to the best of applicants' knowledge is capable of finishing very lightweight webs, nor is any known system capable of dealing with the rapidly changing variations in the position of the repeat pattern on the web and with the problem of cumulative errors of the same type. To date, no known system provides reliable, high quality finishing of previously printed webs, articularly while handling the web sufficiently gently that even lightweight webs can be processed.
It is therefore a principal object of the present invention to provide a registration control arrangement for finishing printed webs which operates on lightweight webs and maintains excellent registration at high speeds despite the presence of both localized and cumulative errors in the position of the impressions.
Another principal object is to provide a registration control system that can operate off-line on pre printed, re-wound webs.
Still another object is to provide a registration control system for off-line finishing of a rewound, pre printed web that imposes no preconditions on the printing for a given repeat length, and therefore can finish any roll printed on any press having the same repeat length.
A further object is to provide a web finishing system that can operate even on very lightweight webs such as tissue used to form airmail envelopes.
Another object is to provide a web finishing system with the foregoing advantages that can operate at high speeds such as the operating speeds of modern printing presses.
Still another object is to provide a system with the foregoing advantages which is characterized by a reduced make ready time and which can be operated independent of a printing press so that the press is productive even during make ready.
Another object is to provide a system with the foregoing advantages which has a favorable cost of manufacture, utilizes many standard components such as known in-line web finishing equipment.